2 Concentration of ions

A strong ionic solute dissociates completely, so ion concentrations are the solute molarity times the number of that ion in the formula.

Worked example 6.4. 0.3 M Al₂(SO₄)₃ → 2Al³⁺ + 3SO₄²⁻. [Al³⁺] = 2 × 0.3 = 0.6 M; [SO₄²⁻] = 3 × 0.3 = 0.9 M.

Worked example 6.5 — volume containing a number of moles. What volume of 0.2 M CaCl₂ contains 1 mol of chloride ions? Each CaCl₂ gives 2 Cl⁻, so [Cl⁻] = 0.4 M. V = 1 / 0.4 = 2.5 L.

Worked example 6.6 — concentration from the ions asked (pattern of the chapter exercise). What volume of 0.3 M Na₂SO₄ must be taken to prepare 2 L of a solution with [Na⁺] = 0.4 M? [Na₂SO₄] needed = 0.4/2 = 0.2 M. M₁V₁ = M₂V₂: 0.3 × V₁ = 0.2 × 2, V₁ = 1.33 L.